.center {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.outer {
    .center;
    width: 200px;
    height: 200px;
    background: lightblue;

    .inner {
        .center;
        width: 100px;
        height: 100px;
        background: lightcoral;
    }
}